Smoke render problem, transparent triangle artifact
Camera Clipping End Distance
One thing to check is to see if your Camera's Clipping End distance is set deep enough to render all parts of your scene. If it is set too close (i.e. to too low of a number), then anything beyond that distance (in Distance units) will not be rendered -- instead, beyond that point, your render will show only the background color, which here is transparent.
To see if this is your problem, check these 2 things:
If this happens when rendering

Select the camera

In the Object Properties window, click on the Camera tab

Increase the Clipping End to a higher number (i.e. farther distance). Here the End distance is set to 100, but you might need to set it higher:


Now render again and see if all of your scene is included in the render.


If this happens when navigating in viewport (i.e. not in camera mode or render)
If this happens when navigating in viewport, then you have to change the clipping end in the Viewport Properties panel:

Press N to open the Viewport Properties

Under View, set the Clip End higher: